The Editors and Readers of COMPUTE!
Too Many Caesars
I own two Commodore computers and a 1541 disk drive. I would like to connect both computers to the drive at once (of course, I would only send disk commands from one computer at a time). Everything works fine when only one computer is turned on, but when I turn on the second one, the first computer does a cold start. When I try to send disk commands from either computer, the entire system seems to lock up. Is there any way to accomplish what I'm trying to do?
Since you can connect more than one peripheral to a single computer, you might expect the reverse to be true. Why can't two computers share the same drive? The answer reveals a fundamental difference between a computer and peripheral devices such as disk drives and printers. The computer is designed to act as "absolute ruler" of the system. It not only sends and receives information (as peripherals can do), but also sends commands that control the whole system. Plugging two computers into the same disk drive is like creating a Rome with two Caesars: Each computer acts like the only commandgiver in existence, and the system becomes confused.
In the first case you describe, turning on the second computer sends a normal reset command to every device in the system-including the second computer, which responds as if it had reset itself. Sending a disk command (which goes to the other computer as well as the drive) makes things even worse. Serial communications require a complex exchange of "handshaking" signals between computer and peripheral to make sure one doesn't send data until the other is ready, and vice versa. Since the second computer isn't designed to respond as a peripheral, it can't complete the handshake and crashes the entire system.
One makeshift way to do what you want is to unplug the serial cable from one computer whenever you want to use the other. However, we definitely don't recommend this as a regular practice. The serial port connectors aren't designed for such heavy use, and you run the risk of sending garbage signals along the line. For long-term use you may want to buy a switching box which cleanly disconnects one computer from the serial bus before connecting the other.